The present invention relates to a method of determining and providing a wear-dependent braking force distribution.
A method for balancing out differences in brake lining wear is described in DE 33 13 078 C2. However the brake linings of the wheel brakes of motor vehicles do not, as a general rule, wear equally rapidly on all the axles or, indeed, on all the wheels as would obviously be desirable for the purpose of maintenance/repair cost optimization. Influencing factors which can cause unbalanced wear are, for example, scatter in the output force of the brake cylinders, in the wheel brake hysteresis, in the effective brake pressure, in the efficiency of the brake application, in the brake characteristics and in the specific wear coefficient.
Methods are known for balancing out differences in brake lining wear between the wheels or axles of motor vehicles braked by pressure medium based either on directly measured wear or residual lining thickness as described in German Patent 33 13 078 or indirectly by way of parameters which determine wear, such as the brake temperature, as described in German Offenlegungsschrift 35 02 051. These methods reduce the brake pressure on the more strongly worn brakes or the brakes with higher thermal loads. Indirect methods, such as the control of the brake pressures with the objective of balanced brake temperatures, have the disadvantage that wear at the same rate cannot be ensured because of the unavoidable scatter in the influencing factors, even if all the brake drums invariably reach the same temperature.
For safety reasons, all known methods of the type mentioned above must be limited to the lowest pressure range, i.e. less than 1 bar, if the use of the frictional adhesion between the tires and the road is not immediately monitored to avoid the danger of locking of the relatively more strongly braked wheels. Although locking is avoided in the case of vehicles with an anti-lock brake system (ABS), however, it still remains undesirable, for reasons of comfort, for the wheel-lock protection, which is intrinsically only intended for limiting cases, to be frequently employed in routine braking simply because of an unsuitable braking force distribution. In the case of the lightly loaded rear axles of empty lorries, on one hand, this would already occur on a wet roadway if pressures are selected for the frequent deceleration levels of 0.1 to 0.25 g largely responsible for wear, where g represents the gravitational acceleration value. On the other hand, a wear control system which only applied in the pressure range below 1 bar would not be functionally reliable because it could only deal with a fraction of the frequent braking actions.
The known solutions also leave the question open as to whether the control objective of "balanced wear" is, where possible, to be achieved in the short term with strong control interventions. Apart from the possible adhesion problems mentioned above, the attainment of this objective would lead to alternate overloading of the respectively less worn brakes and, in consequence, to overall wear which, though balanced, would be premature. Furthermore, the manner in which the magnitude of the brake pressure change necessary is itself to be determined and possibly limited has not been made known.
It is an object of the present invention to prevent adhesion problems without having to be limited to the lowest brake pressures. Wear control is dispensed with only in the case of operationally very rare braking actions above, for example, 0.3*g or a learned adhesion threshold which is limited, in practice, to very critical braking situations. Otherwise, the control interventions are carried out, in accordance with the present invention, with the objective of producing balanced wear only in the long term.
It is yet another object of the present invention to produce wear at the same rate and not just balanced wear.
In the case of electronically controlled brake systems, such as the so-called electro-pneumatic or electro-hydraulic brake systems, it is possible to specify computationally determined pressures deliberately and thereby, to set arbitrary braking force distributions. The computational and storage capability of such known vehicle electronics is used in the present invention. A particularly advantageous embodiment is provided by combination with the optimum adhesion method described in DE 40 07 360 A1.
The present invention is, however, generally suitable for all other methods of determining the basic braking force distribution provided, as in the case with known methods, the wheel rotational speeds are recorded for processing in the anti-lock protection devices.
The method of the present invention is based on the recognition that monitoring of the frictional connection or adhesion below the wheel-lock limit can occur by a simple rotational speed comparison. Determination of the absolute level of the adhesion coefficients or of the slip of the wheels is not necessary for this purpose, and the wheel rotational speeds obtained, for example, by known anti-lock brake system sensors, are sufficient. In this connection, reference is made to the method described in DE 38 29 951 A1. For obtaining balanced wear, a limiting value of the wheel rotational speeds, with reference to the vehicle speed, of, e.g., between 1 and 5%, is usable as a measure of a permissible deviation from that brake pressure distribution which is optimum with respect to the tire/road adhesion. The pressure differences to be selected, which are superimposed if necessary on a basic braking force distribution optimized with respect to adhesion, are limited to a relatively small amount for reasons of comfort. Their magnitude is not changed during a journey, with the exception of conditions critical to adhesion.
In a presently contemplated preferred mode, changes to the wear sensor signals during a journey are ignored. A wear condition is determined at the beginning of each journey and is "frozen". The wear condition is checked, if need be, after a certain number of braking actions for the purpose of excluding measurement errors. Because unbalanced wear can only be recognized after hundreds or even thousands of braking actions even in the case of very fine-resolution sensors, this "freezing" by determining the wear condition essentially only at the beginning of a journey is possible without risk and is also necessary to avoid continually fluctuating control interventions.
According to the present invention, the magnitude of the pressure difference selected is so small that the unbalanced wear only disappears quite slowly over many braking actions. The method of the present invention is, therefore, not a closed-loop control in the usual sense, i.e. no intervention to control wear is provided for the individual braking actions. Because of the high inertia of the system, long-term memories can be appropriate if the electronics are to be provided with a learning character or if measurement scatter in the wear sensing is to be balanced out over several journeys.
Assuming that the wear sensors are sufficiently insensitive to environmental effects (this applies particularly to their dependence on the temperature measurement), the wear values can also be permanently recorded and levelled up in accordance with the method procedure described below. The wear can also be balanced between wheels in a manner analogous to the wear optimization between axles. Independent of the driving condition currently present, equality of pressure between the wheel brakes on one axle is, for this purpose, set as providing optimum adhesion and, on this basis, small pressure differences are fed in when corresponding wear signals are present. The pressure differences possible at the front axle have to be limited because the brake pressure changes necessary for balanced wear can lead to the vehicle pulling to one side.
In addition, the present invention makes it possible to set particular wear ratios as is desirable, for example, in the case of vehicles with a lining volume distribution which departs greatly from the axial load distribution. By way of example, a wear ratio of 2:1 instead of 1:1 can be introduced in a vehicle with disc brakes on one axle and drum brakes on the other so that the drum brakes only have to be relined when, and only precisely when, the disc brakes have worn out two sets of linings.